As is known, many liquid or pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by creasing and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a base layer, e.g. of paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of oxygen-barrier material, e.g. an aluminium foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
Packages of this sort are normally produced on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging unit, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; the web so sterilized is then maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a tube, which is fed along a vertical advancing direction.
In order to complete the forming operations, the tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced transversal cross sections.
Pillow packs are so obtained, which have a longitudinal sealing band, and a top transversal sealing band and a bottom transversal sealing band.
Recently, independently self-movable carriages have been widely applied for conveying packages or opening devices in the packaging industry.
In greater detail, each carriage substantially comprises a respective movable element which is movable along a path and is generally provided with a gripping element. The carriages are magnetically coupled with a stationary magnetic field source, so as to be advanced along the path by the magnetic force associated to the magnetic field.
In particular, it has been proposed, in the embodiment shown in FIG. 17 of WO00/64741, a packaging unit adapted to interact with the tube to heat-seal it at equally spaced cross sections and to form a plurality of pillow packs connected to the tube by transversal sealing bands crosswise to the advancing axis of the tube.
The packaging unit comprises:                a first track and a second track, which are arranged on respective opposite lateral sides of the tube; and        a plurality of first carriages self-movable on the first track and a plurality of respective second carriages self-movable on the second track.        
In particular, the first carriages are movable independently from each other along the first track and the second carriages are movable independently from each another along the second track.
Each first carriage is fitted with a relative sealing element and a relative half-shell, while each corresponding second carriage is fitted with a respective counter-sealing element and with a relative half-shell.
However, the packaging unit shown in FIG. 17 of WO00/64741 leaves room for improvement, especially as regards to the precise and repeatable sealing of the pillow packs in the desired position.
In particular, in case the unit needs to be stooped, the magnetic sources are deactivated, so that the carriages are no longer subjected to any magnetic force.
Accordingly, the movable elements of the carriages arranged along the vertical portion fall, under gravity action, and hit on one another, thus causing an annoying noise and generating a risk of damage.
Carriages are also used in devices for applying glue onto opening device before gluing the latter onto the formed and filled packages.
In this application, the movable elements of the carriages move along a three-dimensional trajectory including also vertical portions under a glue applicator, with the purpose of properly applying the glue on the face of the opening device intended to be glued on the package.
Also in this case, when the magnetic sources are deactivated, the movable element arranged along the vertical portion fall, under gravity action, and hit on one another, thus causing an annoying noise and generating a risk of damage.
A need is felt within the industry to prevent, as far as possible, the fall of the movable elements arranged on the vertical portion of respective paths/trajectory, when the magnetic sources are deactivated.